1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus having a belt-shaped photosensitive body.
The present invention is based on Patent Application No. Hei 10-148680 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
In general, an image forming apparatus of the kind to which this invention relates produces visible images with toner by developing latent images formed in a photosensitive body by the electrophotographic process. These visible images are transferred and fixed by a fixing means from the photosensitive body to a recording medium to form images.
In such image forming apparatuses, it is necessary that the images developed by the photosensitive material be faithful to the latent images, and that transfer from the photosensitive body to the transfer means or to the recording medium be performed without displacement of the images.
In particular, in the case of forming color images by overlapping a plurality of monochromatic images, and in the case of forming compiled images in which drawings, images and maps are attached, where it is necessary to overlap various color images or to insert composite images at the designated position. In such applications, it is necessary to maintain the transfer speed of the belt-shaped photosensitive body constant everywhere on the belt-shaped photosensitive body surface.
When a new latent image is superposed on an already formed image, if the position of the new image is shifted from the required position, good images cannot be obtained.
Moreover, when the belt-shaped photosensitive body is formed in a cylindrical shape, an important requirement is that the surface of the belt-shaped photosensitive body can transfer without causing local fluctuations in the transfer speed, and that the movement of the belt can synchronize with the other elements such as the recording medium.
In, for example, Japanese Patent Application, First Publication No. Hei 5-281810, a technique was proposed to provide a pulley and a belt on the cylindrical belt-shaped photosensitive body which is driven by a driving roller and the resist roller (paper feeding roller), in order to eliminate speed differences between the rotation of the cylindrical belt-shaped photosensitive body and the rotation of the resist roller. Japanese Patent Application, First Publication No. Hei 7-295398 discloses a technique to transfer a driving force from one driving source by means of a series of gears including an eccentric gear for equalizing the peripheral velocities of gears, in order to prevent discrepancies between the rotating speed of the cylindrical belt-shaped photosensitive body and the transfer speed of the transfer belt.
However, when the belt-shaped photosensitive body is used, it is difficult to ensure equal velocity over the whole surface of the belt-shaped photosensitive body, because various guide rollers and fixing means are in contact with the belt-shaped photosensitive body.
When forming a color image by overlapping a plurality of monochromic images, and when forming a compiled image by incorporating color images, graphs, and photographs, these compiled images must be aligned precisely, because displacements of respective objects cannot be allowed. If displacements occur, the obtained images will show color distortion, uneven color, and banding, and be of the low image quality.
Particularly, when the belt-shaped photosensitive body is used, it is preferable to equalize the transfer speed of the belt-shaped photosensitive body over the whole length of the phostosensitive belt by a single driving force, because it is difficult to synchronize a plurality of independent driving sources.
As described hereinabove, when an endless belt type photosensistive belt is used, it is difficult to equalize the transfer speed of the belt-shaped photosensitive body by a single driving source, and to provide an image forming apparatus which is capable of forming images with a high image quality, and this problem hinders the use of endless belt-type photosensitive bodies.
Accordingly, the objects of the present invention are to improve the convenience of the conventional apparatus and to provide an image forming apparatus comprising a driving roller which is in contact with the rear surface of the belt-shaped photosensitive body, one or more guide rollers for guiding the belt-shaped photosensitive body together with the driving roller, and a transmission means for transmitting the driving force of the driving roller directly to at least one of the guide rollers, wherein the image forming apparatus is capable of maintaining the transfer speed of the belt-shaped photosensitive body constant over its whole length and, therefore, it is capable of forming images of excellent quality.
At the same time, it is also an object of the present invention to provide a power-saving type image forming apparatus which is capable of transmitting the driving force of the driving roller efficiently by arranging the guide rollers, photosensitive body, and driving rollers in a characteristic manner for excluding the effects of these elements on the transfer speed of the belt-shaped photosensitive body.